Imaging lens and imaging apparatus

ABSTRACT

An imaging lens includes a first lens, a second lens, a third lens, an aperture stop, and a fourth lens, sequentially arranged from the object-side. The first lens has negative power, and an object-side surface is convex and an image-side surface is concave. Both surfaces of the second lens are aspheric, and in the vicinity of the optical axis, the second lens has negative power, and the object-side surface is concave and the image-side surface is convex. Both surfaces of the third lens are aspheric, and in the vicinity of the optical axis, the third lens has positive power, and the object-side surface is convex, and the image-side surface is convex. Both surfaces of the fourth lens are aspheric, and in the vicinity of the optical axis, the fourth lens has positive power, and the object-side surface is concave, and the image-side surface is convex.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging lens and an imagingapparatus. In particular, the present invention relates to a wide-angleimaging lens that is appropriate for use in an in-vehicle camera, amonitor camera, or the like that uses an imaging device, such as a CCD(Charge Coupled Device) and a CMOS (Complementary Metal OxideSemiconductor). Further, the present invention relates to an imagingapparatus including the imaging lens.

2. Description of the Related Art

In recent years, the size of an imaging device such as a CCD and a CMOSbecame very small, and the resolution of the imaging device became veryhigh. Therefore, the size and the weight of the main body of imagingequipment and an imaging lens mounted on the imaging equipment also needto be reduced. Meanwhile, imaging lenses used in the in-vehicle camera(on-vehicle camera), the monitor camera (or a surveillance camera) andthe like need to have excellent weather-resistance characteristics.Further, the imaging lenses need to have wide angles of view andexcellent optical performance so that an excellent view is ensured for awide range.

Further, since it is desirable to reduce the cost for producing theimaging lens in the aforementioned fields, an optical system composed ofa small number of lenses is needed. Conventionally, for example,Japanese Unexamined Patent Publication No. 2002-244031 (Patent Document1), U.S. Pat. No. 7,280,289 (Patent Document 2), U.S. Pat. No. 7,375,906(Patent Document 3), Japanese Unexamined Patent Publication No.2005-227426 (Patent Document 4), and U.S. Pat. No. 7,518,809 (PatentDocument 5) disclose imaging lenses, each composed of four lenses, inthe aforementioned fields.

Meanwhile, in the fields of the in-vehicle camera, the monitor camera,and the like, a demand for wider angle lenses increased in recent years.For example, lenses having full angles of view exceeding 180° becamedesirable. Further, as the size of the imaging device became smaller,and the resolution of the imaging device became higher in recent years,an imaging lens that has high resolution and high optical performancethat enables obtainment of excellent images for a wide image formationrange became desirable. However, it was difficult for the conventionallens system to satisfy such a demand for an imaging lens that has awider angle of view and high optical performance in recent years, whilethe lens system is constructed at low cost and in small size.

Patent Document 1 describes, as a wide-angle lens in Example 3 thereof,a lens system composed of four lenses of first through fourth lenses,arranged from the object side of the wide angle lens. In the wide anglelens, an aperture stop is arranged between the third lens and the fourthlens. Patent Document 1 is silent about the F-number of the lens systemand the angle of view of the lens system. However, since the refractiveindex of the first lens is approximately 1.52, and the negative power ofthe first lens and the negative power of the second lens are relativelyweak, it is improbable that the lens system satisfies a specification inwhich a full angle of view exceeds 180°.

The full angles of views of the lenses disclosed in Patent Documents 2and 3 are in the range of approximately 140° to 165° and in the range ofapproximately 152° to 164°, respectively. Therefore, they do not copewith a need for a wider angle lens that has a full angle of viewexceeding 180° in recent years. Further, in the lens disclosed in PatentDocument 4, the F-number is in the range of 2.5 to 2.8, and the fullangle of view is greater than or equal to 180°. However, when aprojection method, in which an ideal image height is represented by2×f×tan(φ/2) using focal length f of the entire lens system and halfangle φ of view, is adopted, distortion (distortion aberrations) sharplyincreases on the minus side when the half angle of view exceeds 80°.Therefore, the lens disclosed in Patent Document 4 has a disadvantagethat an image in the most peripheral area thereof becomes small.Further, Patent Document 5 discloses an example in which a full angle ofview is close to 190°. In Patent Document 5, both of the distortion andthe lateral chromatic aberration of the lens are corrected in anexcellent manner. However, astigmatism remains. Further, when the lensis used in combination with a higher-resolution imaging device, a deeperdepth of field is required in some cases.

SUMMARY OF THE INVENTION

In view of the foregoing circumstances, it is an object of the presentinvention to provide a small imaging lens that can be produced at lowcost, but which can realize a wider angle of view and higher opticalperformance. Further, it is another object of the present invention toprovide an imaging apparatus including the imaging lens.

An imaging lens of the present invention is an imaging lens comprising:

a first lens:

a second lens;

a third lens;

an aperture stop; and

a fourth lens, which are sequentially arranged from the object side ofthe imaging lens,

wherein the first lens has negative power, and an object-side surface ofthe first lens is convex and an image-side surface of the first lens isconcave, and

wherein an object-side surface and an image-side surface of the secondlens are aspheric, and the second lens has negative power in thevicinity of the optical axis of the imaging lens, and the object-sidesurface of the second lens is concave in the vicinity of the opticalaxis, and the image-side surface of the second lens is convex in thevicinity of the optical axis, and

wherein an object-side surface and an image-side surface of the thirdlens are aspheric, and the third lens has positive power in the vicinityof the optical axis of the imaging lens, and the object-side surface ofthe third lens is convex in the vicinity of the optical axis, and theimage-side surface of the third lens is convex in the vicinity of theoptical axis, and

wherein an object-side surface and an image-side surface of the fourthlens are aspheric, and the fourth lens has positive power in thevicinity of the optical axis of the imaging lens, and the object-sidesurface of the fourth lens is concave in the vicinity of the opticalaxis, and the image-side surface of the fourth lens is convex in thevicinity of the optical axis.

In the descriptions of the first lens in the imaging lens of the presentinvention, the expression “the first lens has negative power, and anobject-side surface of the first lens is convex and an image-sidesurface of the first lens is concave” refers to a paraxial region of thefirst lens when the first lens is an aspheric lens. Further, theexpression “in the vicinity of the optical axis” is used in the samemeaning as the paraxial region.

In the imaging lens of the present invention, it is desirable that thefollowing formulas (1) through (6) are satisfied:

−12.0<f1/f<−7.0  (1);

0.16<d2/L<0.30  (2);

0.02<d4/L<0.05  (3);

−1.0<f2/f3<−0.4  (4);

4.0<L/f34<6.0  (5); and

−1.0<r5/r4<0.0  (6), where

f: focal length of the entire system of the imaging lens,

f1: focal length of the first lens,

f2: focal length of the second lens,

f3: focal length of the third lens,

f34: combined focal length of the third lens and the fourth lens,

d2: distance between the first lens and the second lens on the opticalaxis,

d4: distance between the second lens and the third lens on the opticalaxis,

r4: paraxial curvature radius of the image-side surface of the secondlens,

r5: paraxial curvature radius of the object-side surface of the thirdlens, and

L: length from the vertex of the object-side surface of the first lensto an image plane.

In the embodiment of the present invention, one of the formulas (1) to(6) may be satisfied. Alternatively, at least two of the formulas (1) to(6) in combination may be satisfied.

In the formulas, the length L uses a back focal length in air. Further,the signs (positive or negative) of the paraxial curvature radii r4, r5are positive when an object-side surface is convex, and the signs of theparaxial curvature radii r4, r5 are negative when an image-side surfaceis convex.

In the imaging lens of the present invention, it is desirable that theAbbe number of the material of the first lens for d-line is greater thanor equal to 40. Further, it is desirable that the Abbe number of thematerial of the second lens for d-line is greater than or equal to 50.Further, it is desirable that the Abbe number of the material of thethird lens for d-line is less than or equal to 40. Further, it isdesirable that the Abbe number of the material of the fourth lens ford-line is greater than or equal to 50.

It is desirable that the imaging lens of the present invention isstructured in such a manner that the full angle of view of the imaginglens is greater than 200°.

An imaging apparatus of the present invention includes an imaging lensof the present invention.

In the imaging lens of the present invention, the power, shape and thelike of each lens are appropriately set in the four-lens optical system,which is composed of four lenses, and an aperture is arranged at anappropriate position. Therefore, it is possible to realize a wider angleof view and higher optical performance at the same time, while theimaging lens is structured at low cost and in small size.

The imaging apparatus of the present invention includes the imaging lensof the present invention. Therefore, it is possible to structure theimaging apparatus at low cost and in small size. Further, the imagingapparatus of the present invention can perform imaging with a wide angleof view, and obtain high-quality images or video images.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an imaging lens in Example 1 of the presentinvention, illustrating the structure of the imaging lens and opticalpaths;

FIG. 2 is a cross section of an imaging lens in Example 2 of the presentinvention, illustrating the structure of the imaging lens and opticalpaths;

FIG. 3 is a cross section of an imaging lens in Example 3 of the presentinvention, illustrating the structure of the imaging lens and opticalpaths;

FIG. 4 is a cross section of an imaging lens in Example 4 of the presentinvention, illustrating the structure of the imaging lens and opticalpaths;

FIG. 5 is a cross section of an imaging lens in Example 5 of the presentinvention, illustrating the structure of the imaging lens and opticalpaths;

FIG. 6 is a cross section of an imaging lens in Example 6 of the presentinvention, illustrating the structure of the imaging lens and opticalpaths;

FIGS. 7A through 7G are diagrams illustrating aberrations of the imaginglens in Example 1 of the present invention;

FIGS. 8A through 8G are diagrams illustrating aberrations of the imaginglens in Example 2 of the present invention;

FIGS. 9A through 9G are diagrams illustrating aberrations of the imaginglens in Example 3 of the present invention;

FIGS. 10A through 10G are diagrams illustrating aberrations of theimaging lens in Example 4 of the present invention;

FIGS. 11A through 11G are diagrams illustrating aberrations of theimaging lens in Example 5 of the present invention;

FIGS. 12A through 12G are diagrams illustrating aberrations of theimaging lens in Example 6 of the present invention; and

FIG. 13 is a diagram for explaining the arrangement of an imagingapparatus for in-vehicle use according to an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to drawings. FIGS. 1 through 6 are cross sectionsof imaging lenses according to embodiments of the present invention.FIGS. 1 through 6 correspond to imaging lenses of Examples 1 through 6,respectively, which will be described later. The examples illustrated inFIGS. 1 through 6 have basically similar structure, and FIGS. 1 through6 are illustrated in a similar manner. Therefore, an imaging lensaccording to an embodiment of the present invention will be describedmainly with reference to FIG. 1.

The imaging lens according to the present embodiment is a lens systemcomposed of four lenses, namely, first lens L1, second lens L2, thirdlens L3 and fourth lens L4, which are sequentially arranged alongoptical axis Z from the object side of the imaging lens. Aperture stop(stop) St is arranged between the third lens L3 and the fourth lens L4.Since the aperture stop St is arranged between the third lens L3 and thefourth lens L4, the size of the imaging lens can be reduced in thedirection of the diameter of the imaging lens.

In FIG. 1, the left side is the object side of the imaging lens, and theright side is the image side of the imaging lens. Further, the aperturestop St illustrated in FIG. 1 does not necessarily represent the sizenor the shape of the aperture stop St, but the position of the aperturestop St on the optical axis Z. In FIG. 1, sign ri (i=1, 2, 3, . . . )represents the curvature radius of each lens surface. Sign di (i=1, 2,3, . . . ) represents a distance (interval) between surfaces. Further,FIG. 1 illustrates an axial ray 2 from an object point at infinity andoff-axial rays 3 at the maximum angle of view.

Further, FIG. 1 illustrates an imaging device 5, which is arranged onimage plane Sim of an imaging lens 1, considering a case of applying theimaging lens 1 to an imaging apparatus. In application of the imaginglens 1 to the imaging apparatus, it is desirable to set a cover glass, alow-pass filter or an infrared-ray cut filter, or the like depending onthe structure of a camera on which the lens is mounted. FIG. 1illustrates a case in which parallel-flat-plate-shaped optical memberPP, assuming such an element, is arranged between the fourth lens L4 andthe imaging device 5 (image plane Sim).

The first lens L1 has negative power. The object-side surface of thefirst lens L1 is convex, and the image-side surface of the first lens L1is concave. The structure of the imaging lens, in which the first lensL1 is a negative meniscus lens having a convex surface facing the objectside as described above, is advantageous to increase the angle of viewof the imaging lens and to correct distortion of the imaging lens. Thefirst lens L1, which is arranged on the most object side of the imaginglens (a side closest to the object), is supposed to be exposed to windand rain or a washing solvent or liquid. Therefore, unwanted particles,dust, water droplets, or the like may remain on the first lens L1.However, if the first lens L1 has meniscus form having a convex surfacefacing the object side, the unwanted particles, dust, water droplets, orthe like does not tend to remain on the first lens L1, and that isadvantageous to the imaging lens.

In the example illustrated in FIG. 1, the first lens L1 is a sphericallens. Alternatively, the first lens L1 may be an aspheric lens. Further,glass is more desirable than resin as the material of the first lens L1,which is arranged on the most object side of the imaging lens, as willbe described later. Therefore, if the first lens L1 is a spherical lens,it is possible to produce the imaging lens at lower cost, compared withthe case of adopting an aspheric lens as the first lens L1.

Both of the object-side surface and the image-side surface of each ofthe second lens L2, the third lens L3, and the fourth lens L4 areaspheric. The second lens L2 has negative power in the vicinity of theoptical axis of the imaging lens, and the object-side surface of thesecond lens L2 is concave in the vicinity of the optical axis, and theimage-side surface of the second lens L2 is convex in the vicinity ofthe optical axis. The third lens L3 has positive power in the vicinityof the optical axis of the imaging lens, and the object-side surface ofthe third lens L3 is convex in the vicinity of the optical axis, and theimage-side surface of the third lens L3 is convex in the vicinity of theoptical axis. The fourth lens L4 has positive power in the vicinity ofthe optical axis of the imaging lens, and the object-side surface of thefourth lens L4 is concave in the vicinity of the optical axis, and theimage-side surface of the fourth lens L4 is convex in the vicinity ofthe optical axis. Since both surfaces of each of the second lens L2, thethird lens L3 and the fourth lens L4 are aspheric, it is possible toachieve high resolution, while reducing the length of the entire opticalsystem in the direction of the optical axis.

In the imaging lens of the present invention, which has4-group/4-element structure, the power and the shape of each of thefirst lens L1 through the fourth lens L4 are appropriately set, and theaperture stop St is arranged between the third lens L3 and the fourthlens L4, as described above. Therefore, a sufficiently wide angle ofview is achieved, while the imaging lens is structured in small size andat low cost by using a small number of lenses. Further, the entirelength of the imaging lens is short. Further, the imaging lens of thepresent invention can correct various aberrations, such as fieldcurvature, distortion, lateral chromatic aberrations, and comaaberrations, in an excellent manner. Since the imaging lens of thepresent invention can achieve high resolution for a wide range of imageformation area, the imaging lens can cope with an imaging device theresolution of which has increased in recent years.

It is desirable that the imaging lens of the embodiment of the presentinvention is further structured as described below. The imaging lens ofthe embodiment of the present invention may have one of the followingstructures. Alternatively, the imaging lens of the embodiment of thepresent invention may have at least two of the structures incombination.

When the focal length of the first lens L1 is f1 and the focal length ofthe entire system of the imaging lens is f, it is desirable that thefollowing formula (1) is satisfied:

−12.0<f1/f<−7.0  (1).

When the value of f1/f exceeds the upper limit defined by the formula(1), the negative power of the first lens L1 becomes strong. Therefore,the absolute value of the curvature radius of the image-side surface ofthe first lens L1 becomes small, and the shape of the image-side surfaceof the first lens L1 becomes close to a hemisphere. Hence, production ofthe first lens L1 becomes difficult, and the cost for producing thefirst lens L1 increases. In contrast, when the value of f1/f becomeslower than the lower limit defined by the formula (1), the negativepower of the first lens L1 becomes weak, and it becomes necessary toincrease the negative power of the second lens L2 to compensate the weakpower of the first lens L1. Consequently, the absolute value of thecurvature radius of the second lens L2 becomes small. Hence, productionof the second lens L2 becomes difficult, and the cost for producing thesecond lens L2 increases.

Further, it is more desirable that the following formula (1-1) issatisfied:

−11.0<f1/f<−7.5  (1-1).

When the formula (1-1) is satisfied, it is possible to increase theadvantageous effect achieved by satisfying the formula (1).

Further, it is even more desirable that the following formula (1-2) issatisfied:

−10.5<f1/f<−7.5  (1-2).

When the formula (1-2) is satisfied, it is possible to further suppressthe cost for producing the imaging lens, compared with the case ofsatisfying the formula (1-1).

When a distance between the first lens L1 and the second lens L2 on theoptical axis is d2, and a length from the vertex of the object-sidesurface of the first lens L1 to an image plane is L, it is desirablethat the following formula (2) is satisfied:

0.16<d2/L<0.30  (2).

Here, the length L uses a back focal length in air.

When the value of d2/L exceeds the upper limit defined by the formula(2), the effective radius of the image-side surface of the first lens L1increases, and becomes close to a curvature radius. Therefore, theprocess for producing the first lens L1 becomes difficult, and the costfor producing the first lens L1 increases. Further, the length of theentire lens system in the direction of the optical axis becomes long. Incontrast, when the value of d2/L is lower than the lower limit definedby the formula (2), if an appropriate power is tried to be ensured inthe first lens L1, the image-side surface of the first lens L1 and theobject-side surface of the second lens L2 interfere with each other.Hence, it becomes impossible to ensure a necessary effective radius, andit becomes difficult to realize an optical system satisfying the objectof the present invention.

Further, it is more desirable that the following formula (2-1) issatisfied:

0.17<d2/L<0.25  (2-1).

When the formula (2-1) is satisfied, it is possible to increase theadvantageous effect achieved by satisfying the formula (2).

When a distance between the second lens L2 and the third lens L3 on theoptical axis is d4 and a length from the vertex of the object-sidesurface of the first lens L1 to an image plane is L, it is desirablethat the following formula (3) is satisfied:

0.02<d4/L<0.05  (3).

Here, the length L uses a back focal length in air.

When the value of d4/L exceeds the upper limit defined by the formula(3), it becomes difficult to correct distortion in an excellent mannerwhile keeping the lateral chromatic aberrations in an excellent manner.Further, the length of the entire lens system becomes long. Theimage-side surface of the second lens L2 and object-side surface of thethird lens L3 should not be in contact with each other in effectivediameter. However, when the value of d4/L becomes lower than the lowerlimit defined by the formula (3), the risk of contact between the twosurfaces increases.

Further, it is more desirable that the following formula (3-1) issatisfied:

0.03<d4/L<0.04  (3-1).

When the formula (3-1) is satisfied, it is possible to increase theadvantageous effect achieved by satisfying the formula (3).

When the focal length of the second lens L2 is f2 and the focal lengthof the third lens L3 is f3, it is desirable that the following formula(4) is satisfied:

−1.0<f2/f3<−0.4  (4).

When the value of f2/f3 exceeds the upper limit defined by the formula(4), distortion and lateral chromatic aberrations increase at middleangles of view. In contrast, when the value of f2/f3 is lower than thelower limit defined by the formula (4), correction of coma aberrationsbecomes difficult. Further, it becomes difficult to suppresslongitudinal chromatic aberrations to a practically applicable level.

Further, it is more desirable that the following formula (4-1) issatisfied:

−0.8<f2/f3<−0.4  (4-1).

When the formula (4-1) is satisfied, it is possible to more easilysuppress coma aberrations and longitudinal chromatic aberrations.

Further, it is even more desirable that the following formula (4-2) issatisfied:

−0.7<f2/f3<−0.5  (4-2).

When the formula (4-2) is satisfied, it is possible to further increasethe advantageous effect achieved by satisfying the formula (4-1).

When the combined focal length of the third lens L3 and the fourth lensL4 is f34 and a length from the vertex of the object-side surface of thefirst lens L1 to an image plane is L, it is desirable that the followingformula (5) is satisfied:

4.0<L/f34<6.0  (5).

Here, the length L uses a back focal length in air.

When the value of L/f34 exceeds the upper limit defined by the formula(5), the power of the third lens L3 becomes weak, and lateral chromaticaberrations are insufficiently corrected, or the power of the fourthlens L4 becomes weak, and field curvature and coma aberrations areinsufficiently corrected. When the value of L/f34 exceeds the upperlimit defined by the formula (5), and the power of the third lens L3 andthe power of the fourth lens L4 are strong, the third lens L3 and thefourth lens L4 are too close to each other, and it becomes difficult toarrange the third lens L3 and the fourth lens L4. Hence, it becomesdifficult to produce the imaging lens at low cost. In contrast, when thevalue of L/f34 is lower than the lower limit defined by the formula (5),the power of the third lens L3 becomes strong, and longitudinalchromatic aberrations become excessively large, or the power of thefourth lens L4 becomes strong, and it becomes difficult to correct fieldcurvature and coma aberrations. When the value of L/f34 is lower thanthe lower limit defined by the formula (5), and neither the power of thethird lens L3 nor the power of the fourth lens L4 is strong, a distancebetween the third lens L3 and the fourth lens L4 becomes long, and thesize of the lens system increases.

Further, it is more desirable that the following formula (5-1) issatisfied:

4.2<L/f34<5.5  (5-1).

When the formula (5-1) is satisfied, it is possible to increase theadvantageous effect achieved by satisfying the formula (5).

When the paraxial curvature radius of the image-side surface of thesecond lens L2 is r4 and the paraxial curvature radius of theobject-side surface of the third lens L3 is r5, it is desirable that thefollowing formula (6) is satisfied:

−1.0<r5/r4<0.0  (6).

When the value of r5/r4 exceeds the upper limit defined by the formula(6), distortion and lateral chromatic aberrations increase at middleangles of view. In contrast, when the value of r5/r4 is lower than thelower limit defined by the formula (6), it becomes difficult to correctcoma aberrations in an excellent manner.

Further, it is more desirable that the following formula (6-1) issatisfied:

−0.5<r5/r4<0.0  (6-1).

When the formula (6-1) is satisfied, it becomes possible to more easilycorrect coma aberrations in an excellent manner.

It is desirable that the Abbe number of the material of the first lensL1 for d-line is greater than or equal to 40. When such material isselected, it becomes possible to easily correct lateral chromaticaberrations in an excellent manner. Further, it is desirable that theAbbe number of the material of the second lens L2 for d-line is greaterthan or equal to 50. When such material is selected, it becomes possibleto easily correct lateral chromatic aberrations in an excellent manner.Further, it is desirable that the Abbe number of the material of thethird lens L3 for d-line is less than or equal to 40. When such materialis selected, it becomes possible to easily correct lateral chromaticaberrations in an excellent manner. Further, it is more desirable thatthe Abbe number of the material of the third lens L3 for d-line is lessthan or equal to 29. When such material is selected, it becomes possibleto easily correct lateral chromatic aberrations in a more excellentmanner. Further, it is desirable that the Abbe number of the material ofthe fourth lens L4 for d-line is greater than or equal to 50. When suchmaterial is selected, it becomes possible to easily correct lateralchromatic aberrations in an excellent manner. Therefore, it is possibleto increase resolution by correcting lateral chromatic aberrations in anexcellent manner. Further, the imaging lens can cope with an imagingdevice, the resolution of which increased in recent years.

Further, it is desirable that the full angle view of the imaging lens ofthe present invention is greater than 200°. The full angle of view istwice the angle formed by the principal ray of the off-axial rays 3 atthe maximum angle of view and the optical axis Z. When the imaging lensis a wide angle lens system having a full angle of view exceeding 200°,it is possible to satisfy the need for wider angle lenses in recentyears.

Further, as in the example illustrated in FIG. 1, it is desirable thateach of all the first lens L1 through the fourth lens L4 is a singlelens, which is not a cemented lens. When an imaging lens is supposed tobe used in tough conditions, for example, in an in-vehicle camera or amonitor camera, it is desirable that no cemented lens is included in theimaging lens. When no cemented lens is included in the imaging lens, itis possible to produce the imaging lens at low cost.

When the imaging lens of the present invention is used in toughconditions, for example, in an in-vehicle camera or a monitor camera,the material of the first lens L1, which is arranged on the most objectside, needs to be resistant to wind and rain, which damages the surfaceof the lens. Further, the material of the first lens L1 needs to beresistant to a change in temperature by direct sunlight. Further, thematerial of the first lens L1 needs to be resistant to chemicals, suchas oils and fats, and detergents. In other words, the material of thefirst lens L1 needs to be highly water-resistant, weather-resistant,acid-resistant, chemical-resistant, and the like. For example, it isdesirable to use a material having water resistance of 1 measured by thepowder method regulated by Japan Optical Glass Industry Standard(JOGIS). Further, in some cases, the material of the first lens L1 needsto be hard and not easily breakable nor crackable. If the material ofthe first lens is glass, it is possible to satisfy such need.Alternatively, transparent ceramic may be used as the material of thefirst lens L1.

Further, a protection means may be provided on the object-side surfaceof the first lens L1 to improve the strength, scratch resistance, andchemical resistance of the surface. In that case, the material of thefirst lens L1 may be plastic. The protection means may be a hard coatingor a water-repellent coating.

It is desirable that the material of the second lens L2, the third lensL3 and the fourth lens L4 is plastic. When the material is plastic, itis possible to accurately form the aspheric shape of each of the lenses.Further, it is possible to reduce the weight of the imaging lens and thecost for production.

Some plastic materials have high water absorption characteristics, andthe refractive indices of such plastic materials and the sizes of shapedplastics may change by absorption or desorption of water, or the like.Consequently, the optical performance may be affected. If plasticmaterials that have extremely low water absorption characteristics areused, it is possible to minimize the deterioration of the performance byabsorption of water. Specifically, polyolefin-based plastic may be usedas the material of the second lens L2 and the fourth lens L4. Further,polycarbonate-based plastic or PET-based(polyethylene-terephthalate-based) plastic may be used as the materialof the third lens L3.

When plastic is used as the material of at least one of the second lensL2, the third lens L3 and the fourth lens L4, so-called nano-compositematerial, in which particles smaller than the wavelength of light aremixed into plastic, may be used.

In the imaging lens of the present invention, an anti-reflection coatingmay be applied to each lens to reduce ghost light or the like. In thatcase, with respect to the image-side surface of the first lens L1, theimage-side surface of the second lens L2, and the object-side surface ofthe third lens L3 of the imaging lens illustrated in FIG. 1, an angleformed by a tangent line at a point in a peripheral area of each of thesurfaces and the optical axis is small. Therefore, the thickness of theanti-reflection coating in the peripheral area is thinner than that ofthe anti-reflection coating in the central area of the lens. Therefore,it is possible to reduce the reflectance of the entire effectivediameter in average by applying, to at least one of the aforementionedthree surfaces, an anti-reflection coating the reflectance of which inthe vicinity of the center is lowest when the wavelength of light isgreater than or equal to 600 nm and less than or equal to 900 nm.Accordingly, ghost light can be reduced.

If the reflectance in the vicinity of the center of the lens is lowestwhen the wavelength of light is less than 600 nm, the wavelength oflight that reflects at the lowest reflectance in the peripheral area ofthe lens becomes too short, and the reflectance for the long wavelengthside becomes high. Therefore, reddish ghost tends to be generated. Incontrast, if the reflectance in the vicinity of the center of the lensis lowest when the wavelength of light is longer than 900 nm, thewavelength of light that reflects at the lowest reflectance in thecentral area of the lens becomes too long, and the reflectance for theshort wavelength side becomes high. Therefore, the color tone (hue) ofan image becomes quite reddish, and bluish ghost tends to be generated.

In the imaging lens of the present invention, a ray of light passing theoutside of the effective diameter between lenses may become stray light,and reach the image plane. Further, the stray light may become ghost.Therefore, it is desirable that a light cutting means for cutting thestray light is provided, if necessary. The light cutting means may beprovided, for example, by applying an opaque paint to a portion of theimage-side surface of the lens, the portion on the outside of theeffective diameter. Alternatively, an opaque plate member may beprovided. The light cutting means may be provided by setting an opaqueplate member in the optical path of rays that will become stray light.

Further, a filter that cuts light in the range of ultraviolet to blue oran IR (InfraRed) cut filter, which cuts infrared light, may be insertedbetween the lens system and the imaging device 5 depending on the use ofthe imaging lens. Alternatively, a coating (coat) that has similarproperties to the aforementioned filters may be applied to the lenssurface.

FIG. 1 illustrates a case in which the optical member PP, assumingvarious filters, is arranged between the lens system and the imagingdevice 5. Instead, the various filters may be arranged between thelenses. Alternatively, a coating that acts in a similar manner to thevarious filters may be applied to a surface of at least a lens in theimaging lens.

Next, examples of numerical values of the imaging lens of the presentinvention will be described. FIGS. 1 through 6 are cross-sections of theimaging lenses in Examples 1 through 6.

Table 1 shows lens data about the imaging lens of Example 1, and Table 2shows aspheric data about the imaging lens of Example 1. Similarly,Tables 3 through 12 show lens data and aspheric data about imaginglenses of Examples 2 through 6. In the following descriptions, themeanings of signs in the tables will be explained by using Example 1.The meanings of the signs are basically the same for Examples 2 through6.

In the lens data of Table 1, column si shows the surface number of thei-th surface (i=1, 2, 3, . . . ). The most object-side surface ofelements constituting the lens system is the first surface, and surfacenumbers sequentially increase toward the image side. Further, column rishows the curvature radius of the i-th surface, and column di shows adistance between the i-th surface and (i+1) th surface on the opticalaxis Z. The sign (positive/negative) of the curvature radius is positivewhen the object-side surface is convex, and negative when the image-sidesurface is convex. In each of the examples, ri and di (i=1, 2, 3, . . .) in the table of lens data correspond to signs ri and di in thecross-section of the lens.

In the lens data of Table 1, column Nej shows the refractive index ofthe j-th lens (j=1, 2, 3, . . . ) for e-line (wavelength is 546.07 nm).The most-object side lens is the first lens, and the number jsequentially increases toward the image side. Further, the column νdjshows the Abbe number of the j-th optical element for d-line (wavelengthis 587.6 nm). The lens data includes aperture stop (stop) St. In thecolumn ri of curvature radii, “∞ (stop)” is written in the boxcorresponding to the aperture stop.

In FIGS. 1 through 6, optical member PP, which is arranged between thefourth lens L4 and image plane Sim, assumes a cover glass, a filter orthe like. In all of Examples 1 through 6, the material of the opticalmember PP is glass having a refractive index of 1.52. The thickness ofthe optical member PP is 1.0 mm in Examples 1 and 2, and the thicknessof the optical member PP is 0.8 mm in Examples 3 through 6.

In the lens data of Table 1, mark “*” is attached to the surface numberof an aspheric surface. Further, Table 1 shows, as the curvature radiusof the aspheric surface, the numerical value of the curvature radius inthe vicinity of the optical axis (paraxial curvature radius). Theaspheric data in Table 2 shows the surface numbers of aspheric surfacesand aspheric coefficients related to the aspheric surfaces. In thenumerical values of aspheric data in Table 2, “E-n” (n: integer) means“×10^(−n)”, and “E+n” means “×10^(n)”. Further, the asphericcoefficients are coefficients K, Bm(m=3, 4, 5, . . . 20) in thefollowing aspheric equation:

Zd=C·h ²/{1+(1−K·C ² ·h ²)^(1/2) }+ΣBm·h ^(m), where

Zd: depth of aspheric surface (the length of a perpendicular from apoint on the aspheric surface at height h to a flat plane that contactswith the vertex of the aspheric surface and is perpendicular to theoptical axis),

h: height (the length from the optical axis to the lens surface),

C: the inverse number of the paraxial curvature radius, and

K, Bm: aspheric coefficients (m=3, 4, 5, . . . 20).

TABLE 1 Example 1 Lens Data si ri di Nej νdj 1 14.2663 1.2000 1.8081146.6 2 4.3500 2.7658 *3 −1.0965 1.1000 1.53619 56.0 *4 −34.3862 0.4439*5 2.1088 2.0602 1.61965 25.5 *6 −17.0613 0.3533 7 ∞(STOP) 0.2674 *8−351.6459 1.5547 1.53619 56.0 *9 −0.9932

TABLE 2 Example 1 Aspheric Data si 3 4 5 6 8 9 K 0 0 0 0 0 0 B33.56918E−01 6.88426E−01 2.46317E−01 −1.82736E−02 −1.40578E−013.79107E−02 B4 −9.45643E−02 −6.29163E−01 −3.20652E−01 2.02512E−011.28976E+00 −5.58640E−01 B5 8.07391E−03 3.15099E−01 3.41421E−01−1.88537E−01 −8.48235E+00 2.22133E+00 B6 −7.25610E−04 4.29952E−016.20134E−02 −1.24348E−01 2.79844E+01 −3.78164E+00 B7 −3.04350E−03−8.69242E−02 −3.74671E−01 4.59906E−01 −5.59117E+01 1.89632E+00 B81.41221E−03 −3.17678E−01 2.84874E−01 −3.55324E−01 1.00825E+029.08799E−01 B9 1.19801E−04 −7.61334E−02 −9.60376E−02 −2.27805E−02−2.93481E+02 9.80625E−01 B10 −4.63764E−05 8.60533E−02 3.32178E−02−1.77836E−01 5.86477E+02 −1.34947E+00 B11 −1.12742E−05 6.04282E−02−2.68752E−02 3.29009E−01 8.28117E+02 −4.77751E+00 B12 −2.93163E−06−1.07082E−02 5.95697E−03 8.92605E−01 −6.70222E+03 6.28669E+00 B13−1.43907E−06 1.56409E−02 −5.29253E−03 −1.47420E+00 1.14500E+04−1.40688E+00 B14 8.96394E−07 −8.10609E−03 5.04349E−03 5.01888E−01−4.60945E+03 −4.67838E−02 B15 −4.46257E−08 −3.08325E−03 3.55531E−03−2.77260E−01 3.68862E+03 −5.61300E−01 B16 3.35128E−08 −6.39513E−031.17605E−03 6.46841E−01 −2.65157E+04 3.21153E−02 B17 2.99449E−091.05157E−03 −2.41742E−03 −6.79400E−01 1.34402E+04 −3.30446E−01 B18−4.34593E−09 9.44835E−04 −2.07694E−04 3.34486E−01 6.29567E+046.04177E−01 B19 −6.68213E−10 1.32306E−03 −5.00700E−04 2.50657E−01−9.02146E+04 −1.75069E−01 B20 2.43160E−10 −5.94765E−04 4.30990E−04−2.27906E−01 3.54494E+04 −1.70929E−02

TABLE 3 Example 2 Lens Data si ri di Nej νdj 1 13.9364 1.1000 1.7762149.6 2 3.6321 2.0526 *3 −1.8912 1.1000 1.53340 55.4 *4 −22.7753 0.4439*5 5.3056 2.1034 1.61965 25.5 *6 −12.5930 0.4507 7 ∞(STOP) 0.2573 *8−52.1131 1.7562 1.53340 55.4 *9 −0.9773

TABLE 4 Example 2 Aspheric Data si 3 4 5 6 8 9 K 0 0 0 0 0 0 B32.77660E−01 7.26447E−01 3.46387E−01 −1.82541E−02 8.75000E−02 8.22827E−02B4 −8.35387E−02 −2.26576E−01 −2.98487E−02 2.58876E−01 −1.55764E+00−3.26258E−01 B5 3.00874E−03 2.73328E−02 −2.09931E−01 −3.11303E−017.60980E+00 5.41264E−01 B6 −1.12619E−03 1.83976E−01 2.02258E−01−3.14633E−02 −9.82974E+00 −4.48097E−01 B7 −1.86753E−03 −1.94793E−022.84071E−02 4.14573E−01 −5.67113E+01 1.79784E−01 B8 1.38107E−03−1.70152E−01 −6.17061E−02 −1.73939E−01 2.05882E+02 −1.52546E−02 B93.11856E−05 −3.27333E−02 −1.83994E−02 −1.23071E−01 −1.55658E+021.15885E−02 B10 −5.31470E−05 4.48618E−02 1.30602E−02 −3.28609E−013.33150E+01 −3.14857E−01 B11 −4.43937E−06 2.98373E−02 1.13508E−032.39474E−01 −2.35347E+01 3.02443E−01 B12 −2.01795E−06 −4.93685E−041.53571E−03 5.53546E−01 −4.15556E+03 1.62270E−01 B13 −1.04287E−06−4.38071E−03 −3.25147E−03 −4.48461E−01 1.28656E+04 −1.24685E−01 B145.12856E−07 −6.18434E−04 −2.72361E−04 −6.99703E−02 −8.05380E+03−1.02095E−01 B15 −1.08659E−08 −1.29888E−03 1.16602E−03 3.25080E−01−3.37008E+03 −1.01761E−01 B16 2.11932E−08 3.40554E−04 1.79989E−03−2.16490E−01 −2.04630E+04 1.42210E−01 B17 3.25461E−09 −3.19194E−04−1.03412E−03 −3.63535E−01 3.45411E+04 5.33285E−02 B18 −2.61678E−09−1.04041E−04 1.20431E−04 3.25671E−01 2.90031E+04 −6.70992E−02 B19−3.80280E−10 3.09509E−04 −3.83503E−04 1.43982E−01 −7.37042E+042.15008E−03 B20 1.16813E−10 −7.67164E−05 1.65494E−04 −1.26405E−013.35000E+04 5.17763E−03

TABLE 5 Example 3 Lens Data si ri di Nej νdj 1 14.0776 1.4000 1.8081146.6 2 4.0025 2.6404 *3 −1.1155 1.1000 1.53619 56.0 *4 −43.3280 0.4439*5 2.3064 1.8987 1.61965 25.5 *6 −8.9295 0.3626 7 ∞(STOP) 0.2585 *8−33.8652 1.5849 1.53619 56.0 *9 −1.0501

TABLE 6 Example 3 Aspheric Data si 3 4 5 6 8 9 K 0 0 0 0 0 0 B33.50520E−01 6.89386E−01 2.50599E−01 −3.85558E−02 −5.69370E−025.95601E−02 B4 −9.46600E−02 −6.54792E−01 −3.37307E−01 2.50063E−019.41342E−01 −6.42505E−01 B5 8.42520E−03 3.28783E−01 3.47086E−01−2.35102E−01 −7.94112E+00 2.38107E+00 B6 −7.08681E−04 4.31004E−016.96931E−02 −1.17675E−01 2.89200E+01 −3.86930E+00 B7 −3.04143E−03−8.50222E−02 −3.74843E−01 5.03085E−01 −5.94045E+01 1.80194E+00 B81.41520E−03 −3.19120E−01 2.86119E−01 −3.79202E−01 1.05389E+021.00347E+00 B9 1.19616E−04 −7.24115E−02 −9.54282E−02 −1.21270E−02−3.02005E+02 1.10411E+00 B10 −4.63866E−05 8.44452E−02 3.06958E−02−2.14406E−01 5.97194E+02 −1.47635E+00 B11 −1.14105E−05 6.18864E−02−2.73543E−02 3.50939E−01 8.26304E+02 −4.78659E+00 B12 −2.90850E−06−1.05242E−02 7.20426E−03 8.57250E−01 −6.70281E+03 6.28034E+00 B13−1.44754E−06 1.57325E−02 −4.91551E−03 −1.49240E+00 1.14657E+04−1.40875E+00 B14 8.98322E−07 −8.17644E−03 5.00384E−03 6.84317E−01−4.45705E+03 −6.67406E−03 B15 −4.67043E−08 −3.09772E−03 3.54308E−03−4.04256E−01 3.53081E+03 −5.86469E−01 B16 3.34461E−08 −6.51372E−031.26393E−03 5.50646E−01 −2.80977E+04 5.26365E−02 B17 3.01197E−091.05006E−03 −2.55410E−03 −6.87376E−01 1.57090E+04 −3.44347E−01 B18−4.31438E−09 9.18380E−04 −2.07164E−04 3.47623E−01 6.60559E+046.49739E−01 B19 −6.60024E−10 1.30044E−03 −5.23098E−04 3.98967E−01−9.76187E+04 −2.34061E−01 B20 2.42820E−10 −5.68323E−04 4.52445E−04−3.11034E−01 3.91046E+04 2.93071E−03

TABLE 7 Example 4 Lens Data si ri di Nej νdj 1 13.9448 1.4500 1.8081146.6 2 3.9000 2.5676 *3 −1.1318 1.1000 1.53619 56.0 *4 −22.4045 0.4439*5 2.3581 1.9311 1.61965 25.5 *6 −10.3718 0.3350 7 ∞(STOP) 0.2576 *8−11.3105 1.6178 1.53619 56.0 *9 −0.9780

TABLE 8 Example 4 Aspheric Data si 3 4 5 6 8 9 K 0 0 0 0 0 0 B33.50293E−01 6.88495E−01 2.39960E−01 −3.62811E−02 −7.49532E−025.78753E−02 B4 −9.50331E−02 −6.65525E−01 −3.21248E−01 2.47845E−011.03678E+00 −6.44424E−01 B5 8.27533E−03 3.24774E−01 3.47948E−01−2.37457E−01 −8.30845E+00 2.39120E+00 B6 −7.18483E−04 4.34609E−016.64840E−02 −1.24161E−01 2.88583E+01 −3.88029E+00 B7 −3.02720E−03−8.06696E−02 −3.76886E−01 5.01945E−01 −5.80778E+01 1.78743E+00 B81.41816E−03 −3.17081E−01 2.85228E−01 −3.78197E−01 1.04067E+029.97396E−01 B9 1.19711E−09 −7.28198E−02 −9.50344E−02 3.40517E−03−2.97427E+02 1.10846E+00 B10 −4.62989E−05 8.37544E−02 3.28838E−02−2.12231E−01 5.89992E+02 −1.46396E+00 B11 −1.14213E−05 6.12424E−02−2.70327E−02 3.42444E−01 7.96747E+02 −4.77657E+00 B12 −2.92420E−06−1.08338E−02 6.61870E−03 8.47297E−01 −6.64156E+03 6.26676E+00 B13−1.45078E−06 1.56752E−02 −5.09373E−03 −1.50636E+00 1.14621E+04−1.40596E+00 B14 8.97169E−07 −8.14160E−03 4.92518E−03 6.73945E−01−4.41729E+03 −9.13554E−03 B15 −4.66823E−08 −3.04633E−03 3.49220E−03−4.05294E−01 3.26781E+03 −5.94009E−01 B16 3.34404E−08 −6.48529E−031.26789E−03 5.61877E−01 −2.78941E+04 5.35659E−02 B17 3.02235E−091.05946E−03 −2.54290E−03 −6.61808E−01 1.62243E+04 −3.34085E−01 B18−4.30950E−09 9.30564E−04 −2.08122E−04 3.68500E−01 6.51072E+046.24717E−01 B19 −6.58396E−10 1.30245E−03 −5.24126E−04 4.10262E−01−9.74122E+04 −2.05863E−01 B20 2.42680E−10 −5.74338E−04 4.60584E−04−3.46981E−01 3.93689E+04 −6.77723E−03

TABLE 9 Example 5 Lens Data si ri di Nej νdj 1 13.6663 1.4500 1.8081146.6 2 3.9000 2.5660 *3 1.1269 1.1000 1.53619 56.0 *4 −15.8198 0.4439 *52.6563 1.9416 1.61965 25.5 *6 −10.7739 0.3517 / ∞(STOP) 0.2569 *8−10.8709 1.5703 1.53619 56.0 *9 −1.0119

TABLE 10 Example 5 Aspheric Data si 3 4 5 6 8 9 K 0 0 0 0 0 0 B33.50046E−01 6.70256E−01 2.33868E−01 −3.79893E−02 −8.35736E−025.62504E−02 B4 −9.50320E−02 −6.57994E−01 −3.19526E−01 2.52881E−011.10723E+00 −6.49599E−01 B5 8.28169E−03 3.27200E−01 3.49161E−01−2.37801E−01 −8.38110E+00 2.39465E+00 B6 −7.21855E−04 4.35574E−016.69890E−02 −1.13480E−01 2.87076E+01 −3.88248E+00 B7 −3.02335E−03−7.89176E−02 −3.75971E−01 4.99517E−01 −5.80029E+01 1.78327E+00 B81.41879E−03 −3.16475E−01 2.85548E−01 −3.76678E−01 1.04215E+029.99006E−01 B9 1.19908E−04 −7.25594E−02 −9.48967E−02 1.21078E−03−2.93883E+02 1.10779E+00 B10 −4.62715E−05 8.37491E−02 3.29607E−02−2.15340E−01 5.86727E+02 −1.45657E+00 B11 −1.14245E−05 6.13251E−02−2.70469E−02 3.38593E−01 7.70696E+02 −4.77587E+00 B12 −2.94183E−06−1.08277E−02 6.58142E−03 8.43077E−01 −6.59811E+03 6.26484E+00 B13−1.45289E−06 1.56342E−02 −5.11710E−03 −1.51157E+00 1.14886E+04−1.40652E−00 B14 8.96749E−07 −8.18186E−03 4.91745E−03 6.68431E−01−4.46654E+03 −1.19097E−02 B15 −4.67614E−08 −3.07765E−03 3.50109E−03−4.13747E−01 3.18865E+03 −5.95465E−01 B16 3.34295E−08 −6.49251E−031.27831E−03 5.76255E−01 −2.77686E+04 5.39051E−02 B17 3.03466E−091.05626E−03 −2.54233E−03 −6.48614E−01 1.63627E+04 −3.29990E−01 B18−4.30474E−09 9.31497E−04 −2.09915E−04 3.76702E−01 6.41944E+046.21301E−01 B19 −6.57461E−10 1.30425E−03 −5.25437E−04 4.13513E−01−9.59376E+04 −2.01529E−01 B20 2.43027E−10 −5.73016E−04 4.60025E−04−3.60332E−01 3.86008E+04 −9.12591E−03

TABLE 11 Example 6 Lens Data si ri di Nej νdj 1 13.4968 1.4500 1.8081146.6 2 3.9000 2.5546 *3 −1.1325 1.1000 1.53619 56.0 *4 −295.3475 0.4439*5 2.3298 1.9680 1.61965 25.5 *6 −13.1515 0.3258 7 ∞(STOP) 0.2529 *8−13.0970 1.6007 1.53619 56.0 *9 −1.0200

TABLE 12 Example 6 Aspheric Data si 3 4 5 6 8 9 K 0 0 0 0 0 0 B33.50176E−01 6.57076E−01 2.30349E−01 −3.50134E−02 −9.56791E−025.85304E−02 B4 −9.50439E−02 −6.33994E−01 −3.20687E−01 2.48995E−011.17835E+00 −6.54408E−01 B5 8.28213E−03 3.15136E−01 3.43573E−01−2.51900E−01 −8.54041E+00 2.40325E+00 B6 −7.72394E−04 4.30677E−016.71486E−02 −9.99637E−02 2.87544E+01 −3.88251E+00 B7 −3.01844E−03−8.21274E−02 −3.71871E−01 4.97335E−01 −5.76360E+01 1.77850E+00 B81.41891E−03 −3.13132E−01 2.86698E−01 −3.74021E−01 1.03585E+029.97807E−01 B9 1.19708E−04 −7.39431E−02 −9.65274E−02 8.46649E−03−2.90244E+02 1.10473E+00 B10 −4.61915E−05 8.44945E−02 3.24985E−02−2.05347E−01 5.71235E+02 −1.45016E+00 B11 −1.13725E−05 6.15078E−02−2.75413E−02 3.22546E−01 7.88479E+02 −4.77498E+00 B12 −2.95151E−06−1.09171E−02 6.53105E−03 8.43757E−01 −6.58648E+03 6.25993E+00 B13−1.44858E−06 1.56145E−02 −5.08989E−03 −1.54124E+00 1.14669E+04−1.40675E+00 B14 8.95862E−07 −8.22721E−03 5.09814E−03 5.74846E−01−4.42248E+03 −4.68099E−03 B15 −4.58710E−08 −3.09665E−03 3.53804E−03−3.07906E−01 2.89534E+03 −5.98035E−01 B16 3.33175E−08 −6.43813E−031.25656E−03 5.90964E−01 −2.72196E+04 5.24730E−02 B17 2.99330E−091.05252E−03 −2.42158E−03 −6.38740E−01 1.61960E+04 −3.28037E−01 B18−4.31528E−09 9.33964E−04 −2.50256E−04 3.96766E−01 6.35735E+046.13332E−01 B19 −6.62175E−10 1.30326E−03 −5.31982E−04 4.28706E−01−9.50938E+04 −1.97328E−01 B20 2.45284E−10 −5.76655E−04 4.44968E−04−4.08952E−01 3.82373E+04 −8.68026E−03

In Examples 1 through 6, the material of the first lens L1 is opticalglass, and both surfaces (the object-side surface and the image sidesurface) of the first lens L1 are spherical. Therefore, the first lensL1 has excellent weather resistant characteristics, and is not easilyscratchable by earth or sand or the like. Further, it is possible toproduce the first lens L1 relatively at low cost. In Examples 1 through6, the material of the second lens L2 and the fourth lens L4 arepolyolefin-based plastic, and the material of the third lens L3 ispolycarbonate-based plastic. The plastics that have low water absorptioncharacteristics are selected so as to suppress fluctuation in theperformance of the lenses by absorption of water as much as possible.

Table 13 shows values corresponding to various data about the imaginglenses of Examples 1 through 6 and formulas (1) through (6). In Examples1 through 6, e-line is reference wavelength, and Table 13 shows valuesat the reference wavelength.

In Table 13, f is the focal length of the entire system of the imaginglens, and Bf is the length from the image-side surface of themost-image-side lens to image plane Sim on the optical axis(corresponding to back focus). Further, L is the length from theobject-side surface of the first lens L1 to the image plane Sim on theoptical axis. Further, Fno. is F-number, and 2ω is the full angle ofview. The value of Bf is a length in air. Specifically, the thickness ofthe optical member PP is converted in air to calculate the back focusBf. Similarly, the back focus portion in the length L is a length inair. In all of Examples 1 through 6, the maximum image height is 1.95mm. As Table 13 shows, all of Examples 1 through 6 satisfy formulas (1)through (6).

TABLE 13 EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 EXAMPLE 6 f0.813 0.845 0.836 0.807 0.829 0.833 Bf 1.872 2.098 2.003 1.995 2.1622.154 L 11.617 11.362 11.692 11.698 11.842 11.850 Fno. 2.8 2.8 2.8 2.82.8 2.8 2ω 221.6° 211.2° 219.0° 219.6° 220.8° 221.6° f1/f −10.075 −7.859−8.825 −8.880 −8.721 −8.731 d2/L 0.238 0.181 0.226 0.219 0.217 0.216d4/L 0.038 0.039 0.038 0.038 0.037 0.037 f2/f3 −0.677 −0.624 −0.681−0.688 −0.638 −0.632 L/f34 4.360 5.208 4.624 4.633 4.814 4.628 r5/r4−0.061 −0.233 −0.053 −0.105 −0.168 −0.008

The values in Table 1 through 13 are rounded to predetermined digits.Further, the unit of the numerical values is “° ” for angles, and “mm”for lengths. However, these units are only examples. Since an opticalsystem can achieve similar optical performance when the optical systemis proportionally enlarged or reduced, other appropriate units may beused.

FIGS. 7A through 7G are diagrams illustrating aberrations of the imaginglens in Example 1. FIGS. 7A through 7D illustrate spherical aberrations,astigmatism, distortion (distortion aberrations), and lateral chromaticaberrations (chromatic aberrations of magnification), respectively.FIGS. 7E through 7G illustrate transverse aberrations in tangentialdirection at respective half angles of view. FIGS. 7A through 7Gillustrate aberrations when e-line is reference wavelength. Further, inFIGS. 7A and 7D, which illustrate spherical aberrations and lateralchromatic aberrations respectively, aberrations for g-line (wavelengthis 435.84 nm) and aberrations for C-line (wavelength is 656.27 nm) arealso illustrated. In the diagram illustrating the spherical aberrations,Fno. represents F-number, and ω in the other diagrams represents halfangles of view.

Similarly, FIGS. 8A through 8G, 9A through 9G, 10A through 10G, 11Athrough 11G, and 12A through 12G illustrate spherical aberrations,astigmatism, distortion (distortion aberrations), lateral chromaticaberrations, and transverse aberrations of imaging lenses in Examples 2through 6, respectively.

In the diagrams illustrating distortion aberrations, shift amounts fromideal image heights “2×f×tan(φ/2)” are illustrated by using focal lengthf of the entire system of the imaging lens and half angle φ of view(variable, 0≦φ≦ω). Therefore, the values are minus in the peripheralarea. However, when the distortions of the imaging lenses in Examples 1through 6 are calculated with respect to image heights based onequidistant projection, the distortions are large plus values. This isbecause the imaging lenses of Examples 1 through 6 are designed so thatimages of the peripheral areas are large, compared with lenses that aredesigned to suppress distortion at image heights based on equidistantprojection.

As the above data show, each of the imaging lenses of Examples 1 through6 is composed of a small number of lenses, i.e., four lenses, therebyreducing the sizes of the imaging lenses and the cost for producing theimaging lenses. Further, the imaging lenses of Examples 1 through 6 havean extremely wide angle of view in the range of 211.2° to 221.6°, and asmall F-number of 2.8. Further, each aberration is corrected in anexcellent manner, and the imaging lenses have high resolution andexcellent optical performance. These imaging lenses are appropriate foruse in monitor cameras and in-vehicle cameras for imaging the frontside, left and/or right side, rear side, or the like of a car.

FIG. 13 illustrates a manner of mounting an imaging apparatus includingthe imaging lens according to the embodiment of the present invention ona car 100. In FIG. 13, an exterior camera 101, an exterior camera 102,and an interior camera 103 are provided in the car 100. The exteriorcamera 101 images the driver's blind spot on a side of the car 100,which is the side of the seat next to the driver. The exterior camera102 images the driver's blind spot on the rear side of the car 100. Theinterior camera 103 is attached to the back side of a rearview mirror,and images the same range as the driver's visual field. The exteriorcamera 101, the exterior camera 102, and the interior camera 103 areimaging apparatuses according to the embodiment of the presentinvention. Each of the exterior camera 101, the exterior camera 102, andthe interior camera 103 includes an imaging lens according to anembodiment of the present invention and an imaging device for convertingan optical image formed by the imaging lens into electric signals.

The imaging lenses according to the embodiments of the present inventionhave the aforementioned advantages. Therefore, the exterior cameras 101and 102, and the interior camera 103 can be structured in small size andproduced at low cost. Further, the exterior cameras 101 and 102, and theinterior camera 103 have wide angles of view, and can obtainhigh-resolution images (or video images) in an excellent manner.

The present invention has been described by using embodiments andexamples. However, the present invention is not limited to theaforementioned embodiments nor examples, and various modifications arepossible. For example, the values of the curvature radius, a distancebetween surfaces, refractive index, Abbe number, and asphericcoefficients of each lens element are not limited to the values in theaforementioned examples of numerical values, but may be other values.Further, the material of the lenses is not limited to the materials usedin the examples of numerical values, but may be other materials.

In the embodiment of the imaging apparatus, a case in which the imaginglens is applied to an in-vehicle camera was described. However, the useof the imaging apparatus of the present invention is not limited to thein-vehicle camera. For example, the imaging apparatus of the presentinvention may be applied to a camera for a mobile terminal, a monitorcamera, and the like.

1. An imaging lens comprising: a first lens: a second lens; a thirdlens; an aperture stop; and a fourth lens, which are sequentiallyarranged from the object side of the imaging lens, wherein the firstlens has negative power, and an object-side surface of the first lens isconvex and an image-side surface of the first lens is concave, andwherein an object-side surface and an image-side surface of the secondlens are aspheric, and the second lens has negative power in thevicinity of the optical axis of the imaging lens, and the object-sidesurface of the second lens is concave in the vicinity of the opticalaxis, and the image-side surface of the second lens is convex in thevicinity of the optical axis, and wherein an object-side surface and animage-side surface of the third lens are aspheric, and the third lenshas positive power in the vicinity of the optical axis of the imaginglens, and the object-side surface of the third lens is convex in thevicinity of the optical axis, and the image-side surface of the thirdlens is convex in the vicinity of the optical axis, and wherein anobject-side surface and an image-side surface of the fourth lens areaspheric, and the fourth lens has positive power in the vicinity of theoptical axis of the imaging lens, and the object-side surface of thefourth lens is concave in the vicinity of the optical axis, and theimage-side surface of the fourth lens is convex in the vicinity of theoptical axis.
 2. An imaging lens, as defined in claim 1, wherein whenthe focal length of the first lens is f1 and the focal length of theentire system of the imaging lens is f, the following formula (1) issatisfied:−12.0<f1/f<−7.0  (1).
 3. An imaging lens, as defined in claim 1, whereinwhen a distance between the first lens and the second lens on theoptical axis is d2, and a length from the vertex of the object-sidesurface of the first lens to an image plane is L, the following formula(2) is satisfied:0.16<d2/L<0.30  (2).
 4. An imaging lens, as defined in claim 1, whereinwhen a distance between the second lens and the third lens on theoptical axis is d4 and a length from the vertex of the object-sidesurface of the first lens to an image plane is L, the following formula(3) is satisfied:0.02<d4/L<0.05  (3).
 5. An imaging lens, as defined in claim 1, whereinwhen the focal length of the second lens is f2 and the focal length ofthe third lens is f3, the following formula (4) is satisfied:−1.0<f2/f3<−0.4  (4).
 6. An imaging lens, as defined in claim 1, whereinwhen the combined focal length of the third lens and the fourth lens isf34 and a length from the vertex of the object-side surface of the firstlens to an image plane is L, the following formula (5) is satisfied:4.0<L/f34<6.0  (5).
 7. An imaging lens, as defined in claim 1, whereinwhen the paraxial curvature radius of the image-side surface of thesecond lens is r4 and the paraxial curvature radius of the object-sidesurface of the third lens is r5, the following formula (6) is satisfied:−1.0<r5/r4<0.0  (6).
 8. An imaging lens, as defined in claim 1, whereinthe Abbe number of the material of the first lens for d-line is greaterthan or equal to 40, and wherein the Abbe number of the material of thesecond lens for d-line is greater than or equal to 50, and wherein theAbbe number of the material of the third lens for d-line is less than orequal to 40, and wherein the Abbe number of the material of the fourthlens for d-line is greater than or equal to
 50. 9. An imaging lens, asdefined in claim 1, wherein the full angle of view of the imaging lensis greater than 200°.
 10. An imaging apparatus comprising an imaginglens as defined in claim 1.